Zoom lens

ABSTRACT

A simple-structured zoom lens having four lens groups is disclosed wherein the zoom lens does not require refocusing after a zooming operation even when the wavelength of light being imaged is different from the design wavelength. The zoom lens includes, in order from the object side, a first lens group which has positive refractive power; a second lens group which has negative refractive power and is moved along an optical axis to accomplish zooming, a third lens group, and a fourth lens group having at least a portion thereof which is moveable along the optical axis during zooming, wherein an image surface is made to form at the same position as for a particular design wavelength when said zoom lens is used to image light at a different wavelength by moving said portion a specified distance in accordance with the wavelength of the incident light and the zoom ratio.

BACKGROUND OF THE INVENTION

Conventionally, a zoom lens is designed in accordance with a usual rangeof wavelengths to be used in imaging an object, and the zoom lens formsan image at a specified image-forming surface at which a detector (e.g.,film or detector array) is positioned. However, there has been a problemin that, when a conventional zoom lens is used outside the range ofwavelengths for which it was designed, the image no longer is formed atthe location of the detector. Therefore, when the zoom lens is usedoutside the range of wavelengths for which it was designed, the imagegets out of focus every time the zoom ratio is changed; thus, refocusingis required.

Some zoom lenses alleviate the inconvenience of having to refocus aftereach zoom operation when the zoom lens is used at another wavelengthrange by moving the film plane or the CCD array surface to adjust forthe different wavelength range. Other zoom lenses alleviate thisinconvenience by inserting a prism between the lens element nearest theimage-side and the image-forming surface in order to adjust the opticalpath length for the change in wavelength range. However, theseadjustments make it more difficult to operate the zoom lens.

Japanese Laid-Open Patent Application No. H8-21943 discloses a zoom lenswhich does not require re-focusing after a zooming operation even whenthe zoom lens is used outside the range of wavelengths for which it wasdesigned. The zoom lens uses a frame of moving patterns whichautomatically adjusts the position of certain lens elements inaccordance with the wavelength of incident light. Thus, re-focusing isnot required every time the zoom ratio is changed despite the zoom lensbeing used for a different range of wavelengths. However, there is aproblem in that the structure of such a zoom lens is complex and twosets of cam grooves are required in order to enable adjustment ofcertain lens elements that are repositioned to adjust for the change inwavelength. Therefore, further resolution of the problem has beensought.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a zoom lens, more particularly, to azoom lens used in a TV camera or a still camera, which can be used formultiple ranges of wavelength, such as for visible light or fornear-infrared light. The object of the present invention is to provide asimple-structured zoom lens which does not require re-focusing everytime a zoom operation is performed, even when the zoom lens is usedoutside its normal wavelength range.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given below and the accompanying drawings, whichare given by way of illustration only and thus are not limitative of thepresent invention, wherein:

FIGS. 1A-1C are schematic diagrams which illustrate an image beingformed at a position P under various conditions,

FIG. 2 shows the lens element configuration of Embodiment 1 of thepresent invention,

FIG. 3 shows the lens element configuration of Embodiment 2 of thepresent invention,

FIGS. 4A-4F show, for near-infrared light having a wavelength of 800 nm,the spherical aberration of the zoom lens of Embodiment 1 for variouszoom ratios before and after movement of a moveable portion of thefourth lens group to correct for the wavelength being different than astandard wavelength (546.1 nm) for which the zoom lens was designed,with FIGS. 4A-4C being before the movement at zoom ratios of 1, 4 and13, respectively, and FIGS. 4D-4F being after the movement at zoomratios of 1, 4 and 13, respectively,

FIGS. 5A-5F show, for near-infrared light having a wavelength of 900 nm,the spherical aberration of the zoom lens of Embodiment 1 for variouszoom ratios before and after movement of a moveable portion of thefourth lens group to correct for the wavelength being different than astandard wavelength (546.1 nm) for which the zoom lens was designed,with FIGS. 5A-5C being before the movement at zoom ratios of 1, 4 and13, respectively, and FIGS. 5D-5F being after the movement at zoomratios of 1, 4 and 13, respectively,

FIGS. 6A-6F show, for near-infrared light having a wavelength of 800 nm,the spherical aberration of the zoom lens of Embodiment 2 for variouszoom ratios before and after movement of the fourth lens group tocorrect for the wavelength being different than a standard wavelength(546.1 nm) for which the zoom lens was designed, with FIGS. 6A-6C beingbefore the movement at zoom ratios of 1, 4 and 10, respectively, andFIGS. 6D-6F being after the movement at zoom ratios of 1, 4 and 10,respectively,

FIGS. 7A-7F show, for near-infrared light having a wavelength of 900 nm,the spherical aberration of the zoom lens of Embodiment 2 for variouszoom ratios before and after movement of the fourth lens group tocorrect for the wavelength being different than a standard wavelength(546.1 nm) for which the zoom lens was designed, with FIGS. 7A-7C beingbefore the movement at zoom ratios of 1, 4 and 10, respectively, andFIGS. 7D-7F being after the movement at zoom ratios of 1, 4 and 10,respectively.

DETAILED DESCRIPTION

As shown in FIGS. 2 and 3, the zoom lens of the present inventionincludes, in order from the object side, a first lens group G₁ which haspositive refractive power, a second lens group G₂ which has negativerefractive power and moves along it s optical axis to accomplishzooming, a third lens group G₃ (which may have either positive ornegative refractive power) and may move along the optical axis tocorrect for what otherwise would be a shifting of the image surface atthe time of zooming, or may be fixed; and a fourth lens group G₄ atleast a portion of which moves with zooming in order to prevent shiftingof the image surface which otherwise would accompany zooming and achange in wavelength from the design wavelength.

A stop 1 may be placed on the object side of the third lens group G₃, orbetween the third lens group G₃ and the object side of the fourth lensgroup G₄, and a filter 2 such as a UV cut-off filter is placed on theimage-side of the fourth lens group G₄. A light beam that enters fromthe object side is formed into an image at image surface 3 that ispositioned along the optical axis X at point P.

As shown in FIGS. 2 and 3, all or a portion of the fourth lens group ofthe present invention is moveable in order to keep the position of theimage surface from shifting due to zooming and the incident light beingat other than the design wavelength. As illustrated in FIG. 3, all ofthe fourth lens group may be moved in order to keep the image surface ata constant position despite zooming and the incident light being atother than the design wavelength. Or, as illustrated in FIG. 2, thefourth lens group G₄ may be divided into two subgroups, G₄a and G₄b,wherein only the object-side subgroup G₄a is moved in order to keep theimage surface at a constant position despite zooming and the incidentlight being at other than the design wavelength.

Referring to FIG. 1A as an example, each lens group G₁-G₄ is arrangedfor a design (i.e., standard) wavelength of 546.1 nm. Thus the zoom lensis designed to focus an object onto a stationary surface (i.e., theimage surface, at which a detector is located) when the incident lighthas a wavelength of 546.1 nm.

As illustrated in FIG. 1B, the image-forming position P shifts to theright for wavelengths that are longer than the design wavelength, withthe position P illustrated being the image point when the incident lighthas a wavelength of 800 nm.

As illustrated FIG. 1C, the image-forming position P can be repositionedto lie at the same position as for the design wavelength. For example,in the case where the fourth lens group is formed of an object-sidesubgroup G₄a and an image-side subgroup G₄b, only a portion of thefourth lens group need be moved toward the image surface in order tostabilize the position of the image surface when the incident light hasa wavelength of 800 nm (as compared to the incident light being at 546.1nm).

There are two methods to move the object-side subgroup G₄a of the fourthlens group G₄. The first method is to employ another cam groove which isdifferent from the cam groove that moves the second lens group G₂ or thefourth lens group G₄ at the time of zooming. The second method is tohave feedback of the distance moved by the zooming operation and toaxially drive the object-side lens subgroup G₄a in order to adjust forthe light being at other than the design wavelength.

For instance, in the case of moving the object-side sub-group G₄a by amotor, the motor can be controlled by a microcomputer whose memory hasdata stored in advance for the moving distances for multiplewavelengths. In this case, a microcomputer functions to adjust themoving distance.

Also, in the case of switching from one incident wavelength to another,there are two methods proposed. A first method is to employ a sensorwhich is capable of detecting the wavelength of incident light on thezoom lens and to automatically switch the position of a compensatinglens component based on the wavelength detected by the sensor. A secondmethod is to manually switch the position of a compensating lenscomponent based on the wavelength range being used based on knowledge bythe operator.

Two specific embodiments of the invention will now be discussed indetail.

Embodiment 1

As shown in FIG. 2, the first lens group G₁ is formed, in order from theobject side, of a first lens element L₁ of negative meniscus shape withits convex surface on the object side, a second lens element L₂ that isbiconvex, and a third lens element L₃ of positive meniscus shape withits convex surface on the object side. The first lens element L₁ and thesecond lens element L₂ are joined.

The second lens group G₂ is formed, in order from the object side, of afourth lens element L₄ that has a negative meniscus shape with itsconvex surface on the object side, a fifth lens element L₅that isbiconcave, and a sixth lens element L₆ that is biconvex. The fifth lenselement L₅ and the sixth lens element L₆ are joined.

The third lens group G₃ is formed, in order from the object side, of aseventh lens element L₇that is biconcave and an eighth lens element L₈that is biconvex. The seventh lens element L₇ and the eighth lenselement L₈ are joined.

The fourth lens group G₄ is formed, in order from the object side, of anobject-side subgroup G₄a and an image-side subgroup G₄b The object-sidesubgroup G₄a is movable along the optical axis X to compensate for achange in zoom ratio as well as for a change in wavelength of theincident light. When light with wavelengths longer than the designwavelength is imaged, the moving member is moved a specified distancetoward the image surface in order to keep the image surface from movingas a result of the change in wavelength.

The object-side subgroup G₄a is formed of, in order from the objectside, a ninth lens element L₉ having a positive meniscus shape with itsconvex surface on the image side, a tenth lens element L₁₀ and aneleventh lens element L₁₁ that are each biconvex, and a twelfth lenselement L₁₂ of negative meniscus shape with its convex surface on theimage side. The eleventh lens element L₁₁ and the twelfth lens elementL₁₂ are joined.

The image-side subgroup G₄b, is formed of, in order from the objectside, a thirteenth lens element L₁₃ and a fourteenth lens element L₁₄that are each biconvex, and a fifteen lens element L₁₅ that isbiconcave. The fourteen lens element L₁₄ and the fifteen lens elementL₁₅ are joined.

Table 1 below lists, for Embodiment 1, the surface number #, in orderfrom the object side, the radius of curvature R (in mm) of each surface,the on-axis surface spacing D, as well as the index of refraction N_(d)and the Abbe number ν_(d) (at the sodium d line) of each lens element.The bottom portion of the table indicates the spacings (in mm) betweenthe lens groups (i.e, the values of D5, D10 and D13) at magnificationratios of 1, 4, and 13, respectively.

TABLE 1 # R D N_(d) V_(d) 1 153.8851 1.8 1.80518 25.4 2 56.822 12.298791.671 57.2 3 −283.192 0.87806 4 53.1924 6.0201 1.6968 55.6 5 147.2384 D5(variable) 6 61.5379 0.9 1.83481 42.7 7 17.3617 5.99065 8 −24.4566 11.804 46.6 9 17.012 4.4668 1.84666 23.9 10 −270.314 D10 (variable) 11−26.1669 1 1.804 46.6 12 59.7 2.48958 1.84666 23.9 13 −240.224 D13(variable) 14 ∞ (stop) 4.18266 15 −214.521 2.25905 1.62041 60.3 16−41.3048 1.12621 17 126.2844 3.69456 1.51742 52.2 18 −69.6535 1.02702 1951.136 5.61984 1.51742 52.2 20 −26.2811 3.28335 1.80518 25.4 21 −113.24529.83082 22 56.4906 2.82022 1.64328 47.8 23 −47.7222 0.08884 24 22.58693.58024 1.51823 59 25 −39.6143 1.82308 1.80518 25.4 26 118.1384 16.75 27∞ 4 1.51633 64.1 28 ∞ 0.20606 29 (image surface) D5 D10 D13 1X 1.02546.032 5.9857 4X 32.949 9.999 10.0954 13X 46.118 4.950 1.975128

Table 2 below lists the amount of axial shift in position (in mm) animage point undergoes when the incident light has a wavelength of 800 nmor 900 nm, as compared to the image position for a wavelength 546.1 nm(the design wavelength) for magnifications 1×, 4× and 13×, respectively.

TABLE 2 800 (nm) 900 (nm) 1X 0.1561 0.2108 4X 0.1775 0.2396 13X 0.32600.4586

Table 3 below lists the distance (in mm) the object-side subgroup G₄a ismoved toward the image surface in order to maintain the image surface atthe position of a design wavelength (i.e., 546.1 nm) when incident lighthas a wavelength of 800 nm or 900 nm, respectively, at magnificationratios of 1×, 4×, and 13×.

TABLE 3 800 (nm) 900 (nm) 1X 0.1796 0.1839 4X 0.1812 0.1862 13X 0.19280.2032

In the present embodiment, as shown above in Table 3, by moving theobject-side subgroup G₄a the distance indicated (which depends on thezoom ratio and the wavelength of the incident light), the image surfaceis maintained at a fixed position despite the incident light being atother than the design wavelength of 546.1 nm.

FIGS. 4A-FIG. 5F show the spherical aberration of the zoom lens ofEmbodiment 1, under various conditions. FIGS. 4A-4F are for the incidentlight having a wavelength of 800 nm. FIGS. 4A-4C show the sphericalaberration without moving the object-side subgroup G₄a, at magnificationratio's of 1×, 4×, and 13×, respectively. FIGS. 4D-4F show the sphericalaberration when the object-side subgroup G₄a is moved the appropriateamount for the incident light having a wavelength of 800 nm, atmagnification ratios of 1×, 4×, and 13×, respectively. FIGS. 5A-5F arefor the incident light having a wavelength of 900 nm. FIGS. 5A-5C showthe spherical aberration without moving the object-side subgroup G₄a, atmagnification ratios of 1×, 4×, and 13×, respectively, and FIGS. 5D-5Fshow the spherical aberration when the object-side subgroup G₄a is movedthe appropriate amount for the incident light having a wavelength of 900nm, at magnification ratios of 1×, 4×, and 13×, respectively.

As clearly indicated from these figures, the spherical aberration issatisfactorily corrected over the entire range of zoom by moving aportion of the fourth lens group G₄ (i.e., subgroup G₄a) in accordancewith the wavelength of the incident light and the zoom ratio.

Embodiment 2

As shown in FIG. 3, the zoom lens of this embodiment includes, in orderfrom the object side, a first lens group G₁ which has positiverefractive power and is fixed in position, a second lens group G₂ whichhas negative refractive power and which moves along the optical axis toaccomplish zooming, a third lens group G₃ which is held stationary atthe time of zooming, and a fourth lens group G₄ which moves along theoptical axis to correct for a shifting of the image surface whichotherwise would occur with zooming and with the incident light having awavelength different from a design wavelength.

An aperture stop 1 is placed between the second lens group G₂ and thethird lens group G₃, and a filter 2 such as a UV blocking filter isplaced on the image side of the fourth lens group G₄. A beam of lightfrom an object is formed into an image on image surface 3 positioned atP along the optical axis X.

The first lens group G₁ includes, in order from the object side, a firstlens element L₁ of negative meniscus shape with its convex surface onthe object side, a second lens element L₂ that is biconvex, a third lenselement L₃ of positive meniscus shape with its convex surface on theobject side. The first lens element L₁ and the second lens element L₂are joined.

The second lens group G₂, includes, in order from the object side, afourth lens element L₄ and a fifth lens element L₅ that are eachbiconcave, and a sixth lens element L₆ that has a positive meniscusshape, with its convex surface on the object side. The fifth lenselement L₅ and the sixth lens element L₆ are joined.

The third lens group G₃ includes, in order from the object side, aseventh lens element L₇ of positive meniscus shape with its convexsurface on the object side.

The fourth lens group G₄, includes, in order from the object side, aneighth lens element L₈ of positive meniscus shape, with its convexsurface on the object side, a ninth lens element L₉ that is biconcaveand a tenth lens element L₁₀ that is biconvex, and an eleventh lenselement L₁₁ that is plano-convex with the convex surface on the objectside. A stop 1 is positioned on the object side of the third lens groupG₃, a filter 2 such as a UV cut-off filter is placed on the image-sideof the fourth lens group G₄, and the image surface 3 is positioned alongthe optical axis X at P.

Table 4 below lists, for Embodiment 2, the surface number #, in orderfrom the object side, the radius of curvature R (in mm) of each surface,the on-axis surface spacing D, as well as the index of refraction N_(d)and the Abbe number V_(d) (at the sodium d line) of each lens element.The bottom portion of the table indicates the spacings (in mm) betweenthe lens groups (i.e, the values of D5, D10, D13 and D21) atmagnification ratios of 1, 4, and 10, respectively.

TABLE 4 # R D N_(d) V_(d) 1 83.3485 2.42 1.84666 23.8 2 38.932 9.271.713 53.9 3 −329.307 0.19 4 33.41 4.94 1.62041 60.3 5 87.8513 D5(variable) 6 −210.767 1.01 1.834 37.2 7 10.3759 4.13 8 −49.962 1.461.51823 58.9 9 10.652 3.62 1.84666 23.8 10 33.6615 D10 (variable) 11 ∞(stop) 1.22402 12 19.6292 2.89 1.744 44.8 13 99.2266 D13 (variable) 1415.6381 3.16 1.62041 60.3 15 191.4499 2.23 16 −22.6145 6.55 1.84666 23.817 15.5523 1.01 18 38.9997 2.68 1.713 53.9 19 −22.4879 0.14 20 16.19432.59 1.755 52.3 21 ∞ D21 (variable) 22 ∞ 5 1.5188 64.2 23 ∞ 5.75563 24 ∞(image) 0 D5 D10 D13 D21 1X 2.509 28.127 3.945 4.000 4X 13.660 16.9762.129 5.816 10X 28.920 1.716 7.612 0.333

Table 5 below lists, for Embodiment 2, the amount of axial shift inposition (in mm) an image point undergoes when the incident light has awavelength of 800 nm or 900 nm, as compared to the image position for awavelength 546.1 nm (the design wavelength) for magnifications 1×, 4×and 10×, respectively.

TABLE 5 800 (nm) 900 (nm) 1X 0.0825 0.1171 4X 0.0678 0.1033 10X 0.36290.5079

Table 6 below lists the distance (in mm) the lens group G₄ is movedtoward the object side in order to maintain the image surface at theposition of a design wavelength (i.e., 546.1 nm) when incident light hasa wavelength of 800 nm or 900 nm, respectively, at magnification ratiosof 1×, 4×, and 10×.

TABLE 6 800 (nm) 900 (nm) 1X −0.0035 −0.0049 4X −0.0028 −0.0042 10X−0.0168 −0.0233

In the present embodiment, as shown above in Table 6, by moving the lensgroup G₄ the distance indicated (which depends on the zoom ratio and thewavelength of the incident light), the image surface is maintained at afixed position despite the incident light being at other than the designwavelength of 546.1 nm.

FIGS. 6A-FIG. 7F show the spherical aberration of the zoom lens ofEmbodiment 2, under various conditions. FIGS. 6A-6F are for the incidentlight having a wavelength of 800 nm. FIGS. 6A-6C show the sphericalaberration without moving the lens group G₄, at magnification ratios of1×, 4×, and 10×, respectively. FIGS. 6D-6F show the spherical aberrationwhen the lens group G₄ is moved the appropriate amount for the incidentlight having a wavelength of 800 nm, at magnification ratios of 1×, 4×,and 10×, respectively. FIGS. 7A-7F are for the incident light having awavelength of 900 nm. FIGS. 7A-7C show the spherical aberration withoutmoving the lens group G₄, at magnification ratios of 1×, 4×, and 10×,respectively, and FIGS. 7D-7F show the spherical aberration when thelens group G₄ is moved the appropriate amount for the incident lighthaving a wavelength of 900 nm, at magnification ratios of 1×, 4×, and10×, respectively.

As is clearly indicated from these figures, the spherical aberration issatisfactory corrected over the entire range of zoom by moving thefourth lens group G₄ in accordance with the wavelength of incident lightand the zoom ratio.

As described above, according to the zoom lens in the present invention,at least a portion of the fourth lens group is a moving member that ismoved axially so that the image forms at a position that is the same asfor a design wavelength despite the lens being used to image light of adifferent wavelength. By moving at least a portion of the fourth lensgroup in accordance with the wavelength of the incident light, there isno need to refocus the system every time a zooming operation isperformed even when the zoom lens is used at a different wavelength.Moreover, a system to move the moving member in accordance with thewavelength of incident light will not be complex, so the production ofthe zoom lens is easy and the cost is reduced.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention. Rather the scopeof the invention shall be defined as set fourth in the following claimsand their legal equivalents. All such modifications as would be obviousto one skilled in the art are intended to be included within the scopeof the following claims.

What is claimed is:
 1. A zoom lens consisting of four lens groups, inorder from the object side, as follows: a first lens group which haspositive refractive power; a second lens group which has negativerefractive power and is moved along an optical axis to accomplishzooming; a third lens group; and a fourth lens group having at least aportion thereof which is moveable along the optical axis during zooming,to thereby cause an image surface to form at the same position as for aparticular design wavelength when said zoom lens is used to image lightat a different wavelength by moving said portion a specified distance inaccordance with the wavelength of the incident light and the zoom ratio.2. The zoom lens of claim 1, wherein the third lens group moves alongthe optical axis to correct a shifting of the image surface whichotherwise would accompany zooming.
 3. The zoom lens of claim 1, whereinthe third lens group is held stationary at the time of zooming.
 4. Thezoom lens of claim 3, wherein the entirety of the fourth lens group ismoveable along the optical axis during zooming.